A work machine such as a hydraulic excavator includes a diesel engine as the prime mover and performs necessary work by driving at least one hydraulic pump with the diesel engine and driving one or more hydraulic actuators with hydraulic fluid delivered from the hydraulic pump. The diesel engine includes a fuel injection unit and controls an engine revolution speed and an output torque by controlling the fuel injection amount by using the fuel injection unit. In the diesel engine of a work machine, to enable various types of work, the output power is generally controlled in such a manner that the power can be outputted in a wide region in an engine revolution speed vs. output torque diagram including a full load characteristic in which the fuel injection amount reaches the maximum (Patent Document 1).
Incidentally, in diesel engines, regulations against the emission of noxious substances contained in the exhaust gas, such as nitrogen oxide (NOx) and particulate matter (PM), is becoming more and more strict in consideration of the influence on human bodies and environment. As methods for reducing these noxious substances, the following two types of methods are known:
(1) A method of improving combustion and thereby reducing the noxious substance emission itself from the engine.
(2) A method of providing a post-treatment device for the exhaust gas from the engine and thereby removing the noxious substances.
In regard to the method (1), there has been proposed, for example, the so-called low-temperature premixed combustion technology that lengthens an ignition delay period by delaying the injection time to a time point after the compression top dead center and improves the combustion by forming premixed gas in the ignition delay period through oxygen concentration reduction by EGR and gas flow control in the combustion chamber (Patent Documents 2 and 3). However, the premixed combustion has technological limitation and its use is limited to a region of low revolution speed and low engine load. In regions where the premixed combustion is unusable, there is no choice but to use diffusive combustion.
In regard to the method (2), there exist a technology of removing particulate matter by using a filter (Patent Document 4), a technology of removing nitrogen oxide by injecting an urea reducing agent or the like into the exhaust gas to cause a reaction (Patent Document 5), and so forth.